1. Field of the Invention
This invention relates to circuit breakers of the type having a bimetallic thermal trip element and, more particularly, to circuit breakers for distribution transformers with indicator light switch mechanism for indicating a current overload condition exceeding a predetermined current value.
2. Description of the Prior Art
Power transformer systems include transformers in association with other protective devices for preventing or limiting power overload damage to the transformer. A completely self-protected transformer includes a current breaker on the secondary or low voltage side to avoid damage due to overload currents. The secondary current breaker disconnects the transformer from its load if the load current exceeds a predetermined rate. Such transformers are disclosed in U.S. Pat. Nos. 3,983,454; 3,997,857; 4,030,053; 4,105,986; 4,119,935; and 4,324,961.
For overload current conditions, it is desirable that the circuit interruption be completed as rapidly as possible after initiation. For the purpose, circuit breakers commonly incorporate a bimetal thermal trip and an instantaneous magnetic trip.
In the overall design of a breaker mechanism, there are various levers and springs which interact with each other to produce the required functions. Inherent in such mechanism designs, there are trade-offs among the interactions. One such trade-off involves three primed forces which act on a trip bar. One of these forces is from the bimetal. When heated, the bimetal pushes the trip bar toward the main contacts in the mechanism. A second force produced by one of the springs is in opposition to the bimetal force and is directed to rotate the trip bar away from the main contacts. The third force is a friction force which resists rotation in either direction. The friction force is between the interface surfaces of a latch cam and a latch plate and the force exists only when the circuit breaker is closed and latched.
In normal operation, the spring force must be great enough to rotate the trip bar away from the main contacts when the breaker is being reset. The bimetal force must be great enough to overcome the sum of the spring force and the frictional force. The frictional force does not have a prime function in normal operation. However, it does enhance stability giving some immunity to vibration and shock. In relative terms, the spring force is less than the friction force and the bimetal force is greater than their sum. Accordingly, there has been a need for a practical combination of these three forces in circuit breaker mechanisms of the type involved.